The invention relates generally to the field of current measurement in power switches, and particularly to load current measurement in switch mode power amplifiers and power supplies.
The subject of the invention is measuring the current flowing through a switch from a power source to a load. Knowledge of the instantaneous amplitude of electrical current flowing to a load is necessary for feedback control systems in switching mode current sources such as the type used to drive three-phase ac motors, or for over-current fault detection in voltage output systems. A very common technique, used particularly in ac motor control, employs a Hall effect sensor on the output line. The Hall probe is relatively expensive, requires complex electronics and exhibits a moderate dc drift term in the output. Another prior art technique uses a sense resistor in series with the output line to the load and a differential amplifier to sense the voltage developed across the resistor. Being in the output line, however, the sense resistor is subjected to a large common mode term from the output voltage. In switching mode systems the switched output voltage means that the differential amplifier needs a very high common rejection ratio from dc to high frequencies. In practice this requirement is costly to achieve.